Commutation signal generators in brushless DC motors commonly employ Hall effect sensors which furnish signals depending upon shaft angle. One or more rotor magnets trigger the Hall sensors which when connected with appropriate circuitry provide the control signals for the switching element of the commutating device. The Hall effect generators are fast, dependable and extremely inexpensive. U.S. Pat. No. 4,283,664 to Ebert, which is hereby incorporated by reference, discloses circuitry which permits two Hall effect sensors displaced 90 degrees apart, electrically, to produce switching signals every 90 degrees electrical.
Furthermore, it is also known to employ stator coil wiring which permits a three-phase stepper motor to achieve 12 torque producing steps within each 360 electrical degrees. U.S. Pat. No. 3,621,358 to Hinrichs et al, which is hereby incorporated by reference, discloses such a wiring circuit which is driven by a clock-pulse generator connected to a shift register and logic gates producing a stepper motor with 12 steps per every 360 electrical degrees.
Problems with the existing art in brushless DC motors include an inherent torque reduction at the time of commutation, friction and magnetic cogging which acting together are referred to as "torque ripple". Most commonly, a three-phase brushless DC motor utilizes only 6 commutation points per 360 electrical degrees and is driven by 3 signaling devices, such as Hall sensors.
One solution to the problem of torque ripple has been the use of sinusoidal current drive systems. However, these are extremely expensive and require a high accuracy shaft angle detector physically built into the motor structure followed by a large amount of accompanying electronic circuitry. It has been found that similar performance can be achieved by the 12-step commutation device disclosed herein at a fraction of the cost.